An AsyncEventListener asynchronously processes batches of events after they have been applied to a region. You can use an AsyncEventListener implementation as a write-behind cache event handler to synchronize region updates with a database.

How an AsyncEventListener Works

An AsyncEventListener instance is serviced by its own dedicated thread in which a callback method is invoked. Events that update a region are placed in an internal AsyncEventQueue, and one or more threads dispatch batches of events at a time to the listener implementation.

You can configure an AsyncEventQueue to be either serial or parallel. A serial queue is deployed to one Geode member, and it delivers all of a region’s events, in order of occurrence, to a configured AsyncEventListener implementation. A parallel queue is deployed to multiple Geode members, and each instance of the queue delivers region events, possibly simultaneously, to a local AsyncEventListener implementation.

While a parallel queue provides the best throughput for writing events, it provides less control for ordering those events. With a parallel queue, you cannot preserve event ordering for a region as a whole because multiple Geode servers queue and deliver the region’s events at the same time. However, the ordering of events for a given partition (or for a given queue of a distributed region) can be preserved.

For both serial and parallel queues, you can control the maximum amount of memory that each queue uses, as well as the batch size and frequency for processing batches in the queue. You can also configure queues to persist to disk (instead of simply overflowing to disk) so that write-behind caching can pick up where it left off when a member shuts down and is later restarted.

Optionally, a queue can use multiple threads to dispatch queued events. When you configure multiple threads for a serial queue, the logical queue that is hosted on a Geode member is divided into multiple physical queues, each with a dedicated dispatcher thread. You can then configure whether the threads dispatch queued events by key, by thread, or in the same order in which events were added to the queue. When you configure multiple threads for a parallel queue, each queue hosted on a Geode member is processed by dispatcher threads; the total number of queues created depends on the number of members that host the region.

A GatewayEventFilter can be placed on the AsyncEventQueue to control whether a particular event is sent to a selected AsyncEventListener. For example, events associated with sensitive data could be detected and not queued. For more detail, see the Javadocs for GatewayEventFilter.

A GatewayEventSubstitutionFilter can specify whether the event is transmitted in its entirety or in an altered representation. For example, to reduce the size of the data being serialized, it might be a more efficient to represent a full object by only its key. For more detail, see the Javadocs for GatewayEventSubstitutionFilter.

Operation Distribution from an AsyncEventQueue

An AsyncEventQueue distributes these operations:

Entry create

Entry put

Entry distributed destroy, providing the operation is not an expiration action

Expiration destroy, if the forward-expiration-destroy attribute is set to true. By default, this attribute is false, but you can set it to true using cache.xml or gfsh. To set this attribute in the Java API, use AsyncEventQueueFactory.setForwardExpirationDestroy(). See the javadocs for details.

These operations are not distributed:

Get

Invalidate

Local destroy

Region operations

Expiration actions

Expiration destroy, if the forward-expiration-destroy attribute is set to false. The default value is false.

Guidelines for Using an AsyncEventListener

Review the following guidelines before using an AsyncEventListener:

If you use an AsyncEventListener to implement a write-behind cache listener, your code should check for the possibility that an existing database connection may have been closed due to an earlier exception. For example, check for Connection.isClosed() in a catch block and re-create the connection as needed before performing further operations.

Use a serial AsyncEventQueue if you need to preserve the order of region events within a thread when delivering events to your listener implementation. Use parallel queues when the order of events within a thread is not important, and when you require maximum throughput for processing events. In both cases, serial and parallel, the order of operations on a given key is preserved within the scope of the thread.

You must install the AsyncEventListener implementation on a Geode member that hosts the region whose events you want to process.

If you configure a parallel AsyncEventQueue, deploy the queue on each Geode member that hosts the region.

You can install a listener on more than one member to provide high availability and guarantee delivery for events, in the event that a member with the active AsyncEventListener shuts down. At any given time only one member has an active listener for dispatching events. The listeners on other members remain on standby for redundancy. For best performance and most efficient use of memory, install only one standby listener (redundancy of at most one).

Install no more than one standby listener (redundancy of at most one) for performance and memory reasons.

To preserve pending events through member shutdowns, configure Geode to persist the internal queue of the AsyncEventListener to an available disk store. By default, any pending events that reside in the internal queue of an AsyncEventListener are lost if the active listener’s member shuts down.

To ensure high availability and reliable delivery of events, configure the event queue to be both persistent and redundant.

Implementing an AsyncEventListener

To receive region events for processing, you create a class that implements the AsyncEventListener interface. The processEvents method in your listener receives a list of queued AsyncEvent objects in each batch.

Each AsyncEvent object contains information about a region event, such as the name of the region where the event occurred, the type of region operation, and the affected key and value.

The basic framework for implementing a write-behind event handler involves iterating through the batch of events and writing each event to a database. For example:

Processing AsyncEvents

Use the AsyncEventListener.processEvents method to process AsyncEvents. This method is called asynchronously when events are queued to be processed. The size of the list reflects the number of batch events where batch size is defined in the AsyncEventQueueFactory. The processEvents method returns a boolean; true if the AsyncEvents are processed correctly, and false if any events fail processing. As long as processEvents returns false, Geode continues to re-try processing the events.

You can use the getDeserializedValue method to obtain cache values for entries that have been updated or created. Since the getDeserializedValue method will return a null value for destroyed entries, you should use the getKey method to obtain references to cache objects that have been destroyed. Here’s an example of processing AsyncEvents:

Configuring an AsyncEventListener

To configure a write-behind cache listener, you first configure an asynchronous queue to dispatch the region events, and then create the queue with your listener implementation. You then assign the queue to a region in order to process that region’s events.

Procedure

Configure a unique AsyncEventQueue with the name of your listener implementation. You can optionally configure the queue for parallel operation, persistence, batch size, and maximum memory size. See WAN Configuration for more information.

If you are using a parallel AsyncEventQueue, the gfsh example above requires no alteration, as gfsh applies to all members. If using cache.xml or the Java API to configure your AsyncEventQueue, repeat the above configuration in each Geode member that will host the region. Use the same ID and configuration settings for each queue configuration.
Note:
You can ensure other members use the sample configuration by using the cluster configuration service available in gfsh. See Overview of the Cluster Configuration Service.

On each Geode member that hosts the AsyncEventQueue, assign the queue to each region that you want to use with the AsyncEventListener implementation.

gfsh Configuration

gfsh>create region --name=Customer --async-event-queue-id=sampleQueue

Note that you can specify multiple queues on the command line in a comma-delimited list.

Optionally configure persistence and conflation for the queue.
Note:
You must configure your AsyncEventQueue to be persistent if you are using persistent data regions. Using a non-persistent queue with a persistent region is not supported.